The present invention relates generally to medical devices and particularly to a catheter used to implant stents which has a permanently affixed conductor.
The use of stents to treat various organs, such as the vascular system, colon, biliary tract, urinary tract, esophagus, trachea and the like, has become common. Typically, stents are useful in treating blockages, occlusions, narrowing ailments and other similar problems that restrict flow through a passageway.
One common medical treatment in which stents are used involves implanting an endovascular stent into the vascular system. Stents are useful for numerous medical treatments of various vessels throughout the vascular system, including both coronary vessels and peripheral vessels (e.g., carotid, brachial, renal, iliac and femoral). However, the use of stents in coronary vessels has drawn particular attention from the medical community due to the commonality of heart problems caused by stenosis (i.e., narrowing of a vessel).
Although stenosis may occur for a variety of reasons, one of the most common causes of coronary stenosis results from the buildup of atherosclerotic plaques along the lumen of the vessel. The resulting coronary stenosis restricts blood flow through the vessel, which eventually can lead to a dangerously increased risk of heart attacks.
The medical community has attempted to address coronary stenosis (along with the many other passageway problems that patients suffer from) with various versions of percutaneous transluminal angioplasty (“PTA”). Fundamentally, PTA involves inserting a balloon-tipped catheter into a vessel and threading the catheter to the narrowed portion to be treated. The balloon is then expanded at the narrowed portion by pumping saline through the catheter to the balloon. As a result, the balloon expands, contacts the inner vessel wall, and forces the vessel to dilate. The balloon is then deflated and retracted from the vessel.
One problem that has been encountered with typical PTA procedures is restenosis (i.e., re-narrowing) of the vessel. Restenosis may occur for a variety of reasons, such as collapsing of the vessel wall or growth of cellular tissue. For example, restenosis may occur due to damage caused to the vessel lining during balloon expansion and vessel dilating. As a result of the damage caused to the intima layers of the vessel, the vessel attempts to grow new intima tissue to repair the damage. This tendency of vessels to regrow new tissue is referred to as neointimal hyperplasia. The effect of this response results in a re-narrowing of the vessel. However, restenosis is not completely predictable and may occur either abruptly soon after the PTA procedure due to vessel collapse or may occur slowly over a longer period of time due to other reasons.
One approach the medical community has tried in order to overcome the problems with restenosis is to use stents in conjunction with the above-described PTA procedure. Traditionally, stents are made of metal or other synthetic materials, thereby providing a tubular support structure that radially supports the inner wall of the vessel. Although other materials are possible and are sometimes used, the most common materials now used in stents are stainless steel (e.g., 316L SS and 304 SS) and Nitinol. Typically, stents are designed with a pattern of openings formed in the support structure that permits the stent to radially expand from a small diameter to a larger diameter. Accordingly, stents are now commonly used in conjunction with conventional PTA procedures by positioning the stent within the portion of the vessel that has been dilated by the balloon and radially expanding the stent against the inner wall of the vessel to permanently implant the stent. The expectation of this revised PTA procedure is that the support structure of the implanted stent will mechanically prevent the vessel from collapsing back to its original narrowed condition.
Although stent designs and implantation procedures vary widely, two categories are common.
The first of these two categories may be referred to as balloon-expandable stents. Balloon-expandable stents are generally made from ductile materials that plastically deform relatively easily. In the case of stents made from metal, 316L stainless steel that has been annealed is a common choice for this type of stent. One common procedure for implanting a balloon-expandable stent involves mounting the stent circumferentially on the balloon prior to threading the balloon-tipped catheter to the narrowed vessel portion that is to be treated. When the balloon is positioned at the narrowed vessel portion and expanded, the balloon simultaneously dilates the vessel and also radially expands the stent into the dilated portion. The balloon and the catheter are then retracted, leaving the expanded stent permanently implanted at the desired location. Ductile metal lends itself to this type of stent since the stent may be compressed by plastic deformation to a small diameter when mounted onto the balloon. When the balloon is then expanded in the vessel, the stent is once again plastically deformed to a larger diameter to provide the desired radial support structure. Traditionally, balloon-expandable stents have been more commonly used in coronary vessels than in peripheral vessels due to the deformable nature of these stents. One reason for this is that peripheral vessels tend to experience frequent traumas from external sources (e.g., impacts to a person's arms, legs, etc.) which are transmitted through the body's tissues to the vessel. In the case of peripheral vessels, there is an increased risk that an external trauma could cause a balloon-expandable stent to once again plastically deform in unexpected ways with potentially severe and/or catastrophic results. In the case of coronary vessels, however, this risk is minimal since coronary vessels rarely experience traumas transmitted from external sources.
A second common category of stents is referred to as self-expandable stents. Self-expandable stents are generally made of shape memory materials that act like a spring. Typical metals used in this type of stent include Nitinol and 304 stainless steel. A common procedure for implanting a self-expandable stent involves a two-step process. First, the narrowed vessel is dilated with the balloon as described above. Second, the stent is implanted into the dilated vessel portion. To accomplish the stent implantation, the stent is installed on the end of a catheter in a compressed, small diameter state and is retained in the small diameter by inserting the stent into the lumen of the catheter or by other means. The stent is then guided to the balloon-dilated portion and is released from the catheter and allowed to radially spring outward to an expanded diameter until the stent contacts and presses against the vessel wall. Traditionally, self-expandable stents have been more commonly used in peripheral vessels than in coronary vessels due to the shape memory characteristic of the metals used in these stents. One advantage of self-expandable stents for peripheral vessels is that traumas from external sources do not permanently deform the stent. Instead, the stent may temporarily deform during an unusually harsh trauma but will spring back to its expanded state once the trauma is relieved. Self-expandable stents, however, are often considered to be less preferred for coronary vessels as compared to balloon-expandable stents. One reason for this is that balloon-expandable stents can be precisely sized to a particular vessel diameter and shape since the ductile metal that is used can be plastically deformed to a desired size and shape. In contrast, self-expandable stents are designed with a particular expansible range. Thus, after being installed self-expandable stents continue to exert pressure against the vessel wall.
However, even when a stent is used in conjunction with conventional PTA procedures, restenosis still remains a problem. As discussed above, one cause of restenosis is neointimal hyperplasia which may result from damage to the vessel wall. This cause of neointimal hyperplasia remains a problem even when a stent is used. In addition, the synthetic materials that are usually used in stents may also contribute to neointimal hyperplasia. The cause of this problem is the body's tendency to grow new living tissues around and over newly implanted foreign objects. Thus, despite the mechanical support structure provided by the stent, restenosis remains a problem.
One approach that has been offered to address the problem of restenosis has been to coat stents with drugs that are designed to inhibit cellular growth. Although many such drugs are known, common examples of these types of drugs include Paclitaxel, Sirolimus and Everolimus. However, despite the benefits of these types of drugs, numerous problems still exist with the current methods that are used to apply these and other coatings to stents.
Typically, a stent is provided by the manufacturer as part of a pre-assembled package. For example, in the case of a balloon-expandable stent, the package may include a catheter, a balloon formed at the end of the catheter, and a drug-coated stent mounted onto the balloon. In the case of a self-expandable stent, the package may include a catheter, a mounting apparatus for retaining and releasing the stent, and a drug-coated stent mounted on the apparatus.
One method that may be used to manufacture the above-described stent assemblies involves coating the stent first in a separate step and then mounting the coated stent onto the balloon or other mounting apparatus. Common coating processes include dipping, spraying and painting the drug onto the stent. However, these methods suffer from numerous problems. One problem is the difficulty of mounting the coated stent onto the catheter without damaging the coating that has been applied. In addition, many conventional coating processes are difficult to control and apply an uneven coating on the stent. Moreover, when the stent is coated separately, it is difficult to avoid coating at least part of both the inside and outside surfaces of the stent.
Alternatively, the stent may be coated after being mounted onto the catheter. However, this method has not been perfected. As mentioned, conventional coating processes are difficult to control and apply uneven coatings. This can be an even more significant problem when the coating is applied to a stent mounted to a catheter since the coating inevitably ends up coating the catheter, balloon and/or mounting apparatus also.
Precisely controlling the application of coatings on stents is important for a number of reasons. For example, in the case of drug coatings in particular, it is important to ensure that the drug is applied as evenly as possible on the specific surfaces where the coating is needed. This ensures a uniform physiological response to the drug after the stent is implanted. Another important reason for precisely controlling the application of coatings is the high cost of many coatings. Typically, in the case of drug coatings, the cost of the drug per unit volume can be very expensive. Therefore, the drug should be applied as precisely as possible to the surfaces of the stent where the effectiveness of the drug can be maximized. Thus, by minimizing waste during the coating process, the overall cost of the stent assembly may be reduced.
It is apparent to the inventor that a stent assembly is desired in which coatings may be applied to the stent in a more effective manner than is presently possible. Accordingly, a solution is described more fully below that solves this and other problems.